Such devices as are the subject of the invention are used, among others, in the automobile industry to produce functional automobile body parts by joining sheet metal parts having different or identical thicknesses. The properties of the parts such as, for example, their surface coating, can be different. If possible, the welding seams created with the device should not be thicker than the metal sheets, so that any further processing by stamping or cold forming is not impaired.
Therefore, a laser device is used as the welding source for the butt-joined metal sheets, which are essentially up to 3 mm thick. When laser radiation is used for welding, a very narrow welding seam is formed with almost no seam elevation and the surrounding material heats up less than with conventional processes. Moreover, the required properties can be achieved at high welding speeds. For this reason, laser devices are used more and more frequently to join bands or metal sheets (European Patent No. 0,151,848 B1, European Patent No. 0,299,358 A1, West German Patent No. 3,502,368 A1).
However, the use of a laser device as the welding source calls for a precise preparation of the welding seam. The permissible joint between the parts to be joined depends on the thickness of the metal sheets and it must be &lt;0.1 mm in the case of thin parts. The position tolerance of the butt joint must also be &lt;0.1 mm. An effort is made to try to fulfill these requirements by using shears with blades on both sides in order to cut the ends, of the bands or metal sheets. While the ends are being cut, the bands or metal sheets are held between clamping elements.
European Patent No. 0,151,848 B1 proposes shears that can be moved laterally into and out of the welding area to cut the ends, whereby the ends are held between the lower blades and the clamping elements. After the cutting step, that is, after the shears have been moved out of the welding area, the ends are clamped between the clamping elements and a backing strip which can be moved horizontally.
The result is a long cutting cycle time due to the shears which can be moved in and out laterally, and due to the subsequent separate movement of the backing strip.
This cutting cycle time can be reduced by means of shears disclosed in West German Patent No. 3,044,350 C3, which are attached to a carriage in such a way that they can pivot; the height of the carriage, in turn, can be adjusted on a holding beam. This achieves maximum accessibility to the working space during the welding procedure without the shears having to be moved in and out. A backing strip is attached to the pivoting shears and this backing strip is fixed in its working position when the shears are in their uppermost position.
Moreover, West German Patent No. 1,116,034 C3 discloses a saw to cut the ends of bands and a welding torch as the welding source.
It has also already been proposed to use shearing blades and to combine them with a backing strip to form an integrated component (West German Patent No. 1,216,073, Swiss Patent No. 390,658). In this construction, the shearing blades and the backing strip rest upon an upper cross beam supported by two vertical beams that can be moved up and down together, whereby the shearing blades are located on the bottom side of the upper cross beam. After the seam has been made, the upper cross beam with the shearing blades is positioned underneath the band level, so that the backing strip is now underneath the band ends which are to be welded together.
In this known embodiment, it is no longer possible, for example, to cut the band once again immediately after a flawed welding step by means of the shearing blades positioned underneath, and for this reason, this embodiment has never been implemented in actual practice. Moreover, it is also disadvantageous that the band ends are not clamped onto the backing strip but rather between upper and lower pairs of clamping jaws.